The present invention relates to a propulsion device for vehicles based on a diesel internal combustion engine, in which a fuel to be burnt is initially partially oxidized and then burnt as a mixture with the unoxidized fuel in the diesel internal combustion engine. The present invention furthermore relates to a method for operating a diesel internal combustion engine using an at least partially oxidized diesel fuel and to the use of a partially oxidized diesel fuel to reduce soot emissions in the above-described propulsion device.
Strategies for reducing soot emissions in diesel vehicles, especially fine soot particles, are currently the subject of intensive research. Not least of the reasons for this are the Euro V and Euro VI exhaust standards, which require significant reductions in soot emissions.
One starting point for reducing soot particle emissions which has established itself in connection with diesel vehicles in the last few years, and was stipulated by the Euro VI standard, is the use of diesel particle filters. Here, diesel soot particles are collected in a filter system and are then converted to gaseous decomposition products (substantially CO2) by heating to a temperature above 250° C. Filtering the particulate exhaust gas constituents is not the actual problem. It is rather the regeneration of this diesel particle filter as completely as possible, in which the soot particles collected must be broken down into gaseous decomposition products.
In the case of regeneration, a distinction is drawn between passive continuous regeneration and active discontinuous regeneration. Passive continuous regeneration takes place at a temperature above 250° C. and preferably involves NO2 formed over an oxidation catalyst in accordance with the reaction formula2NO2+C→CO2+2NO.
In addition, discontinuous active regeneration, preferably with oxygen at a temperature of more than 450° C., must take place at least at certain intervals in accordance with the reaction formulae shown below.C+O2→CO2 2C+O2→2CO.
For active regeneration with oxygen, it is generally necessary to raise the exhaust gas temperature. This can be accomplished by measures applied to the engine, such as intake air throttling, or afterinjection and measures downstream of the engine, such as a burner, external fuel metering or external heated catalyzers. However, there is the problem with measures applied to the engine and downstream of the engine that the particle filter has to be heated to a high temperature for active combustion, to achieve which the vehicle must be driven for a certain distance. If vehicles are not driven over relatively long distances, unburnt soot residues in the filter can lead to problems, especially blockage of the filter. Moreover, measures downstream of the engine are often associated with an increased energy requirement, resulting in higher fuel consumption by the vehicles.
These problems have led to an increasing search for approaches with which it is possible to suppress the formation of soot in diesel engines a priori.
A relevant approach described in EP 0 590 914 consists in mixing an additive into the fuel from an additional tank before combustion. This additive lowers the burn-off temperature of the soot particles in the particle filter and thus facilitates the combustion thereof.
In addition, it is possible to use oxygen-containing “fuel additives” to lower the regeneration temperature. The addition of fuel additives is described in DE 10 2009 035 503 A1 or DE 10 2008 032 254 B4, for example. In US 2006/180099 A1 or WO 2011/126653 A1, oxygen-containing constituents, such as ethanol, are added to the diesel fuel, in which the ethanol contained in the fuel is subjected to acid-catalyzed dehydration before combustion. The diethyl ether thereby formed has significantly better combustion properties than ethanol, and therefore the combustion process can be controlled with the aid of the generation of the ether. However, one problem associated with adding ethanol to diesel fuel is that the two constituents can dissociate in the tank, making it necessary to add further additives to stabilize the mixture. These, in turn, require expensive production and distribution, leading to considerable additional costs for the fuel.
An alternative approach is followed by Bromberg, L. et al. in his contribution entitled “On-Board Plasmatron Generation of Hydrogen Rich Gas for Diesel Engine Exhaust Aftertreatment and Other Applications”, PSFC JA-02-30, of 11 Dec. 2002. In this approach, a plasmatron reformer is used to convert diesel fuel or bio oils into a hydrogen-rich gas. Diesel plasmatron reformer technology is capable of converting significant amounts of fuel into hydrogen without the need for a catalyst. This is achieved through the use of a special plasma for the non-catalytic conversion of the diesel fuel into hydrogen, with the hydrogen yields being sufficient to produce an NOX trap.
The conversion of fuel by means of a plasma was also described in EP 1 891 309 B1, in which some of the fuel carried in the vehicle was subjected to treatment with an ozone-containing air plasma. For this purpose, the volatile constituents of the diesel were evaporated by introducing a gas and made to react with the ozone. The decomposition products from the diesel which were formed in this reaction were then passed into the exhaust gas flow in order to facilitate the afterburning of the soot particles there.
DE 103 59 395 A1 also describes afterburning of fuel residues after they leave the combustion chamber, in which a reducing gas, such as hydrogen, carbon monoxide, ammonia or hydrocarbons or mixtures thereof, is introduced into the exhaust gas flow. For this purpose, the reducing gas is first of all produced by means of an “on-board” reactor from the fuel being carried, wherein the reactor is designed in such a way that a reforming-splitting or a cracking process occurs. By means of this process, some of the fuel is converted into said reducing agents, which are then introduced into the exhaust line for reductive ash decomposition.
However, one disadvantage of measures which start after the actual decomposition of the fuel is that the amount of soot which is originally formed is not affected, i.e. that combustion itself cannot be improved. However, exhaust gas aftertreatment is always less advantageous in terms of energy than control of combustion in such a way that the amount of soot which then has to be burnt at additional expense is less.
Finally, DE 23 65 255 discloses a method for reducing nitrogen oxide emissions from petroleum combustion processes, wherein some of the fuel to be fed to the internal combustion engine is converted into decomposition and oxidation products, such as aldehydes and ketones, carbon monoxide, hydrogen and short-chain hydrocarbons in a reformer and is then fed to an internal combustion engine. In this case, the mixture supplied is introduced into the combustion chamber of the engine particularly in the region of the spark plug and serves as a highly combustible charge (“rich mixture”). This mixture is ignited by the spark plugs, from where combustion propagates to the remainder of the fuel mixture (“lean mixture”). However, DE 23 65 255 relates essentially to conventional spark ignition engines and to the reduction of nitrogen oxide emissions.